The objective was to flush out militant Sikhs who were fighting for an independent homeland in Punjab.

Bluestar achieved its military objectives - but its costs were enormous. Although there has never been any agreement on the numbers killed, Sikhs say thousands were massacred by Indian troops, many of them innocent bystanders.

The military action led to Mrs Gandhi's assassination later that year by her Sikh bodyguards. That in turn triggered a wave of anti-Sikh rioting which left nearly 3,000 dead.

Jagtar Singh of the UK Sikh Federation, said the arrival of a new Indian government, now led by a Sikh prime minister, had given hope to the Diaspora that the truth could eventually come out.

Sunday's march and rally in central London, which was expected to be attended by a number of MPs and London mayoral candidates, included demands on the British government to pressure Delhi to open up the episode to full international scrutiny.

"This temple is the holiest of holies," Mr Singh told the BBC. "And while trying to flush out a small number of militants, the Indian army killed enormous numbers of innocent pilgrims.

"During the operation there was a complete media blackout in Punjab. But 20 years later and the blood still cannot be washed from the marble floors."

Mr Singh said that while the attack had shocked Sikh communities around the world, it had also led to a reawakening of identity.

"People [in the UK] who had drifted away from their faith start practising it again," he said. "India actually laid the foundation stones for an independence movement and a more aware Sikh community."

Hopes on new PM

He said British Sikhs were hoping that the new Indian prime minister Manmohan Singh would open up the past to "put the record straight".

He was already recognised as a "straight politician" - but the community wanted to see him act as an "honest man".

This could be done by allowing Amnesty International, the United Nations and other organisations fully investigate the events of 1984.

Sikhs campaigning over the Golden Temple attack have long enjoyed the support of many MPs and they want more pressure put on the Foreign Secretary to speak out over what happened.

Mr Singh said: "All we say is let the truth come out. Let people in the international community judge what happened."